1. Field of the Invention
The present invention relates to a pixel circuit having an organic electroluminescence (EL) element or other electro-optic element with a luminance controlled by a current value and an image display device comprised of such pixel circuits arrayed in a matrix, in particular a so-called active matrix type image display device controlled in value of current flowing through the electro-optic elements by insulating gate type field effect transistors provided inside the pixel circuits, and a driving method of the pixel circuits.
2. Description of the Related Art
In an image display device, for example, a liquid crystal display, a large number of pixels are arranged in a matrix and the light intensity is controlled for every pixel in accordance with the image information to be displayed so as to display an image. This same is true for an organic EL display etc. An organic EL display is a so-called self light emitting type display having a light emitting element in each pixel circuit and has the advantages that the viewability the image is higher in comparison with a liquid crystal display, a backlight is unnecessary, the response speed is high, etc. Further, it greatly differs from a liquid crystal display etc. in the point that the gradations of the color generation are obtained by controlling the luminance of each light emitting element by the value of the current flowing through to, that is, the light emitting element is a current controlled type.
An organic EL display, in the same way as a liquid crystal display, may be driven by a simple matrix and an active matrix system, but while the former has a simple structure, it has the problem that realization of a large sized and high definition display is difficult. For this reason, much effort is being devoted to development of the active matrix system of controlling the current flowing through the light emitting element inside each pixel circuit by an active element provided inside the pixel circuit, generally, a thin film transistor (TFT).
FIG. 1 is a block diagram of the configuration of a general organic EL display device. This display device 1 has, as shown in FIG. 1, a pixel array portion 2 comprised of pixel circuits (PXLC) 2a arranged in an m×n matrix, a horizontal selector (HSEL) 3, a write scanner (WSCN) 4, data lines DTL1 to DTLn selected by the horizontal selector 3 and supplied with a data signal in accordance with the luminance information, and scanning lines WSL1 to WSLm selectively driven by the write scanner 4.
FIG. 2 is a circuit diagram of an example of the configuration of a pixel circuit 2a of FIG. 1 (refer to for example U.S. Pat. No. 5,684,365 and Japanese Unexamined Patent Publication (Kokai) No. 8-234683. The pixel circuit of FIG. 2 has the simplest circuit configuration among the large number of proposed circuits and is a so-called two-transistor driving system circuit.
The pixel circuit 2a of FIG. 2 has a p-channel thin film field effect transistor (hereinafter, referred to as TFT) 11 and TFT 12, a capacitor C11, and a light emitting element made of an organic EL element (OLED) 13. Further, in FIG. 2, DTL indicates a data line, and WSL indicates a scanning line. An organic EL element has a rectification property in many cases, so sometimes is referred to as an organic light emitting diode (OLED). The symbol of a diode is used as the light emitting diode in FIG. 2 and the other figures, but a rectification property is not always required for an organic EL element in the following explanation. In FIG. 2, a source of the TFT 11 is connected to a power supply potential VCC, and a cathode of the light emitting diode 13 is connected to a ground potential GND. The operation of the pixel circuit 2a of FIG. 2 is as follows.
Step ST1
When the scanning line WSL is made a selected state (low level here) and a write potential Vdata is supplied to the data line DTL, the TFT 12 becomes conductive, the capacitor C11 is charged or discharged, and the gate potential of the TFT 11 becomes Vdata.
Step ST2
When the scanning line WSL is made a non-selected state (high level here), the data line DTL and the TFT 11 are electrically separated, but the gate potential of the TFT 11 is held stably by the capacitor C11.
Step ST3
The current flowing through the TFT 11 and the light emitting diode 13 becomes a value in accordance with a gate-source voltage Vgs of the TFT 11, while the light emitting diode 13 is continuously emitting light with a luminance in accordance with the current value. As in the above step ST1, the operation of selecting the scanning line WSL and transmitting the luminance information given to the data line to the inside of a pixel will be referred to as “writing” below. As explained above, in the pixel circuit 2a of FIG. 2, if once the Vdata is written, the light emitting diode 13 continues to emit light with a constant luminance in the period up to the next rewriting.
As explained above, in the pixel circuit 2a, by changing a gate application voltage of the drive transistor constituted by the TFT 11, the value of the current flowing through the EL light emitting element 13 is controlled. At this time, the source of the drive transistor of p-channel is connected to the power supply potential VCC, so this TFT 11 is always operating in a saturated region. Accordingly, it becomes a constant current source having a value shown in the following equation 1.Ids=½·μ(W/L)Cox(Vgs−|Vth|)2  (1)
Here, μ indicates the mobility of a carrier, Cox indicates a gate capacitance per unit area, W indicates a gate width, L indicates a gate length, Vgs indicates the gate-source voltage of the TFT 11, and Vth indicates the threshold value of the TFT 11.
In a simple matrix type image display device, each light emitting diode emits light only at a selected instant, while in an active matrix, as explained above, the light emitting element continues emitting light even after the end of the writing. Therefore, it becomes advantageous in especially a large sized and high definition display in the point that the peak luminance and peak current of the light emitting element can be lowered in comparison with a simple matrix.
However, TFTs generally exhibit large variation in the Vth and mobility μ. For this reason, even if the same input voltage is supplied to the gates of different drive transistors, the on current thereof will vary. As a result, the uniformity of the image quality will deteriorate.
In order to alleviate this problem, a large number of pixel circuits have been proposed. A typical example is shown in FIG. 3 (refer to for example U.S. Pat. No. 6,229,506 and Japanese National Publication (Tokuhyo) No. 2002-514320).
A pixel circuit 2b of FIG. 3 has p-channel TFT 21 to TFT 24, capacitors C21 and C22, and a light emitting element made of an organic EL light emitting diode (OLED) 25. Further, in FIG. 3, DTL indicates a data line, WSL indicates a scanning line, AZL indicates an auto-zero line, and DSL indicates a drive line.
An explanation will be given below of the operation of this pixel circuit 2b while referring to the timing charts shown in FIGS. 4A to 4G. FIG. 4A shows a scanning signal ws[1] applied to the scanning line WSL1 of the first row of the pixel array; FIG. 4B shows a scanning signal ws[2] applied to the scanning line WSL2 of a second row of the pixel array, FIG. 4C shows an auto-zero signal az[1] applied to the auto-zero line AZL1 of the first row of the pixel array; FIG. 4D shows an auto-zero signal az[2] applied to the auto-zero line AZL2 of the second row of the pixel array; FIG. 4E shows a drive signal ds[1] applied to the drive line DSL1 of the first row of the pixel array; FIG. 4F shows a drive signal ds[2] applied to the drive line DSL2 of the second row of the pixel array; and FIG. 4G shows a gate potential Vg of the TFT21. Note that, the operation of the pixel circuit of the first row will be explained below.
As shown in FIGS. 4C and 4E, the drive signal ds[1] to the drive line DSL1 and the auto-zero signal az[1] to the auto-zero line AZL1 are made the low level, and the TFT 22 and TFT 23 are made the conductive state. At this time, the TFT 21 is connected to the light emitting element (OLED) 25 in a diode-connected state, so the current flows through the TFT 21. At this time, the gate potential Vg of the TFT 21 falls as shown in FIG. 4G.
As shown in FIG. 4E, the drive signal ds[1] to the drive line DSL1 is made the high level, and the TFT 22 is made the non-conductive state. At this time, when the scanning signal ws[1] to the scanning line WSL1 is the high level, the TFT 24 is held in the non-conductive state as shown in FIG. 4A. Along with the TFT 22 becoming the non-conductive state, the current flowing through the light emitting element 25 is shut off, therefore, as shown in FIG. 4G, the gate potential Vg of the TFT 21 rises, but the TFT 21 becomes the non-conductive state and the potential becomes stable at the point of time when the potential rises up to Vcc−|Vth|. This operation will be referred to as an “auto-zero operation”.
As shown in FIG. 4C, the auto-zero signal az[1] to the auto-zero line AZL1 is made the high level and the TFT 23 is made the non-conductive state to terminate the auto-zero operation (Vth correction operation), then the drive signal ds[1] to the drive line DSL1 is made the low level to make the TFT 22 the conductive state.
Then, the scanning signal ws[1] to the scanning line WSL1 is made the low level as shown in FIG. 4A.to make the TFT 24 is made the conductive state and a data signal having a predetermined potential propagated through the data line DTL1 is applied to the capacitor C21. Due to this, as shown in FIG. 4G, the gate potential of the TFT 21 is lowered by exactly ΔVg via the capacitor C21. As shown in FIG. 4A, the scanning line WSL1 is made the high level to make the TFT 24 the non-conductive state. Due to this, the current flows through the TFT 21 and the EL light emitting element (OLED) 25, and the EL light emitting element 25 starts to emit light.
Summarizing the problems to be solved by the invention as explained above, in the pixel circuit of FIG. 3, by turning on the auto-zero switch constituted by the TFT 23 during a period when the EL light emitting diode 25 does not emit light, the drive transistor TFT21 is made a cut-off state. In the cut-off state, no current flows through this transistor TFT 21, so the gate-source voltage Vgs thereof becomes equal to the threshold value Vth of each transistor, and the Vth variation for every pixel is cancelled. Next, by turning off the TFT 23, then turning on the TFT 24, a voltage ΔV is coupled with the gate of the drive transistor TFT21 through the capacitor C21 in the pixel of the data line voltage. Assuming that this coupling amount is V0, the drive transistor TFT 21 will not depend upon the Vth, an on current corresponding to Vgs−Vgh=V0 flows, and an image quality without unevenness of uniformity due to Vth variation is obtained.
In the pixel circuit of FIG. 3, however, even if the Vth variation can be corrected, the variation of the mobility μ cannot be corrected. Below, this problem will be explained in further detail in relation to the drawings.
FIG. 5 is a graph of characteristic curves of ΔV (=Vgs−Vth) of drive transistors having different mobilities and the drain-source current Ids in the pixel circuit of FIG. 3. In FIG. 5, an abscissa represents the voltage ΔV, and an ordinate represents the current Ids. Further, in FIG. 5, a curve indicated by a solid line indicates the characteristic of a pixel A, and a curve indicated by a broken line indicates the characteristic of a pixel B.
As shown in FIG. 5, the mobility is different between the characteristic of the pixel A indicated by the solid line and the characteristic of the pixel B indicated by the broken line. In the pixel circuit system of FIG. 3, at the auto-zero point (ΔV=V0), the current value is equal even between pixel transistors having different mobilities. However, as the voltage rises thereafter, the variation of the mobility μ appears in the current value. For example, in the pixel A and the pixel B having different mobilities, even when the same voltage ΔV=V0 is applied, variation of the current Ids occurs according to the above equation 1 and the luminances of the pixels become different. That is, a large current flows, the current value ends up being affected by the variation of the mobility as it becomes bright, the uniformity varies, and the image quality ends deteriorating.
Further, FIG. 6 is a graph of the change of the gate voltage of the drive transistor at the time of an auto-zero operation at pixels C and D having different threshold values Vth of the drive transistor. In FIG. 6, the abscissa represents the time t, and the ordinate represents the gate voltage Vgs. Further, in FIG. 6, a curve indicated by the solid line indicates the characteristic of a pixel C, and a curve indicated by the broken line indicates the characteristic of a pixel D.
The auto-zero operation is carried out by connecting the gate and the source of the drive transistor. Also, the on current thereof rapidly decreases as it approaches the cut-off region. For this reason, a long time is required until the variation of the cut-off threshold value is completely cancelled. As shown in FIG. 6, when the auto-zero time is insufficient, the variation of the threshold value Vth is not completely cancelled in the pixel C. In this way, due to the variation of the threshold value Vth, it is also believed that variation occurs even in the writing state of the gate voltage and therefore the uniformity is deteriorated due to this.
Further, even if the variation of the threshold value Vth is cancelled by taking sufficient time for the auto-zero operation, an off current will flow through the drive transistor after the cut-off, though small in amount. For this reason, as shown in FIG. 7, the gate voltage gradually rises toward the power supply voltage Vcc. As a result, regardless of the fact that variation of the threshold value Vth was once cancelled by the auto-zero operation, the gate potentials of the pixels having the threshold value Vth variation finally become uniform toward the power supply voltage, so the variation of the threshold value Vth appears again.
From the above description, in order to effectively cancel the variation of the threshold value Vth in an actual device, it is necessary to optimally adjust the auto-zero period for every panel. However, this adjustment of the optimum auto-zero period for every panel takes an enormous amount of time and raises the cost of the panels.